1. Field of the Invention
The present invention relates to a modulator, specifically to a modulator for use in radio equipment.
This application is a counterpart of Japanese patent application, Serial Number 284537/2001, filed Sep. 19, 2001, the subject matter of which is to incorporated herein by reference.
2. Description of the Related Art
Conventionally, a modulator by the so-called direct modulation system has a configuration illustrated in FIG. 7. Here, the direct modulation system directly applies a modulating signal, such as a data pulse train, etc., to voltage controlled reactance elements inside a resonance circuit included in a Voltage Controlled Oscillator 720 (hereunder, simply referred to as “VCO”) being a constituent of the modulator.
In such a modulator as shown in FIG. 7, the resonance circuit included in the VCO 720 possesses two pairs of the voltage controlled reactance elements such as varactor diodes, or the like. And, to one of them, to the varactor diode 721 is applied the control voltage outputted from a Phase-Locked Loop (hereunder, simply referred to as “PLL”) circuit 710 for setting a carrier frequency, as the dc bias voltage; and to the other varactor diode 722 is applied a modulating signal 730 as the dc bias voltage.
Generally, the modulation factor in the modulator as shown in FIG. 7 is defined by the following expression.Modulation Factor=Frequency Deviation/(Modulating signal Data Rate/2)  (1)
Here, the denominator of the expression (1) represents the frequency of the 3modulating signal. On the other hand, the numerator of the expression (1), the frequency deviation is given by the following.ΔC2 /(C1+C2)  (2)In the expression (2), C1 and C2 signify the capacitance values in each of the series branches of the varactor diodes VD1 and VD2, and ΔC2 represents the capacitance variation of the capacitance C2.
Now, assuming that the setting of the carrier frequency is made changed in the modulator in FIG. 7, it is natural that the capacitance C1 of the varactor diode 721 for setting the carrier frequency varies to follow the control voltage outputted from the PLL 710. In contrast to this, the capacitance C2 of the varactor diode 722 for the direct modulation is almost constant, being proportional to the mark rate of the transmission data as the modulating signal. For example, when the carrier frequency is raised, the control voltage from the PLL 710 is also increased, which decreases the capacitance C1 of the varactor diode 721. The reason is as follows. The varactor diode controls the thickness of the depletion layer produced on the PN junction by the dc bias voltage applied, and thereby achieves the variable reactance characteristic. Therefore, if the dc bias voltage is increased, the reverse field on the PN junction is strengthened to widen the depletion layer, whereby the capacitance C1 formed by the depletion layer is to be decreased. On the other hand, the capacitance C2 of the varactor diode 722, as mentioned above, is in irrelevance with the carrier signal; accordingly, the capacitance C2 remains constant even with the variation of the carrier frequency.
In other words, varying the frequency of the carrier signal varies the value of C1 in the expression (2), which gives an influence to the value of the frequency deviation, and the variation of the frequency deviation leads to a variation of the modulation factor given by the expression (1). In general, the modulation factor in the modulator represents the depth of modulation of the modulating signal against the carrier signal. Therefore, if the modulation factor varies, the distribution of the frequency band and frequency spectrum in possession of the carrier signal having the modulation applied, which is the output signal of the modulator, will vary, thus leading to apprehensions that the demodulation on the receiver side cannot be made smoothly.